The present invention relates to a method of driving a liquid-crystal display mounted on projection displays, view finders, head-mount displays, etc.
Active-matrix displays are usually driven by an analog signal that controls a drive voltage for liquid crystals, such as, disclosed in Japanese Unexamined Patent Publication No. 11-174410 (1999).
There are several modes for liquid crystals:
1. Polarization Mode
Ferroelectric Liquid Crystal (FLC);
Vertical Aligned (VA);
Hybrid Aligned Nematic (HAN);
Twisted Nematic (TN);
Electrically Controlled Birefringence (ECB);
Mixed-mode Twisted Nematic (MTN);
Self-Compensated Twisted Nematic (SCTN);
Reflected Twisted Nematic (RTN); and
Hybrid Field-Effect (HFE)
2. Dispersion Mode
Polymer Dispersed Liquid Crystal (PDLC)
3. Diffraction Mode
Zero Field Diffraction (ZFD)
Liquid crystals used in high picture-qualify systems are VA, MTN, etc. Particularly, VA is used for obtaining high contrast ratio.
Active-matrix displays have multiple pixels formed with a liquid crystal filled between an active-matrix substrate and another substrate. A signal supplied to each pixel is stored in a storage capacitor provided for the pixel, to drive the liquid crystal.
FIG. 1 shows a schematic block diagram of a typical active-matrix display driven by an analog signal.
In FIG. 1, column-signal electrodes D1, D2, D3, . . . , and Di are aligned on an active-matrix substrate 2. Also aligned on the substrate 2 are row-scanning electrodes G1, G2, G3, . . . , and Gj, intersecting with the column-signal electrodes.
Provided at the intersection of each column-signal electrodeD (D1, D2, D3, . . . , and Di) and each row-scanning electrode G (G1, G2, G3, . . . , and Gj) is a pixel Px having a pixel-switching transistor Tr, a storage capacitor Cs, and a liquid crystal LC, as shown in FIG. 2.
A column-signal-electrode driver 100 is equipped with a horizontal shift register 101 and several analog switches S1, S2, S3, . . . , and Si.
Input terminals of the analog switches S1, S2, S3, and Si are connected to a display-signal supply line L through which a display signal VIDEO is supplied. Output terminals of the switches S1, S2, S3, . . . , and Si are connected to the column-signal electrodes D1, D2, D3, . . . , and Di, respectively. A control terminal of each switch S is connected to the corresponding output of the horizontal shift register 101.
The horizontal shift register 101 is driven by a horizontal start signal HST and a horizontal clock signal HCK, to output pulses. The signals HST and HCK are supplied from a drive timing pulse generator (not shown).
The pulses output from the are horizontal shift register 101 are supplied to the analog switches S1, S2, S3, . . . , and Si, to sequentially turn on these switches. The turn-on switches allow the display signal VIDEO for one horizontal period to be sequentially supplied to the column-signal electrodes D1, D2, D3, . . . , and Di.
A row-scanning-electrode driver 102 is equipped with a vertical shift register having several register stages corresponding to the number of rows to be displayed.
The vertical shift register is driven by a vertical start signal VST and a vertical shift clock signal VCK synchronizing with one horizontal period, to output scanning pulses. The signals VST and VCK are supplied from a drive timing pulse generator (not shown).
The scanning pulses output from the vertical shift register are sequentially supplied to the row-scanning electrodes G1, G2, G3, . . . , and Gj per horizontal period (per row).
A vertical period of the display signal VIDEO is synchronized with the vertical start signal VST.
The scanning pulses turn on, sequentially per row, the pixel-switching transistors Tr connected to the row-scanning electrodes G1, G2, G3, . . . , and Gj.
Each turned-on pixel-switching transistor Tr in FIG. 2 allows the display signal VIDEO, supplied to the corresponding column-signal electrode D, to be stored as charge information in the storage capacitor Cs of the corresponding pixel Px.
The stored charge information is supplied to the liquid crystal LC via a display-pixel electrode 20 for light modulation. The light modulation provides images to be displayed corresponding to the display signal VIDEO.
This type of active-matrix display, however, has the following disadvantages.
In FIG. 2, the scanning pulse is supplied to the gate of the pixel-switching transistor Tr to turn on the transistor to store the charge information (display signal VIDEO) in the storage capacitor Cs.
At the moment of supplying the scanning pulse (gate voltage Vg) to the gate of the pixel-switching transistor Tr, a voltage (drain voltage Vd) appearing at the drain of the transistor Tr rapidly varies as shown in FIG. 3 (field-through voltage Vft) due to field through caused by a gate-to-drain floating capacitance CGD, with respect to a voltage Vf of an electrode 21 facing the display-pixel electrode 20 in FIG. 2.
Subsequent no supply of the scanning pulse to the gate of the pixel-switching transistor Tr varies the drain voltage and then keeps the varied drain voltage, as shown in FIG. 3.
The voltage varying as shown in FIG. 3 is then supplied to the liquid crystal LC, as D.C. (direct current) components, which causes low photo response and image persistence (or burn-in), etc, thus resulting in short life for the liquid-crystal panel.
In addition, this type of active-matrix display is prone to generation of noises on the display signal VIDEO and effects of pseudo display signals, although provides enhanced gradation with voltages supplied to the liquid crystal constant for one-field period but varying in accordance with the level of the display signal VIDEO.
A method of driving a liquid crystal with pulses is disclosed in Japanese Unexamined Patent Publication No. 2001-166749, to solve the problems discussed above.
In this method, pixels are turned on or off in accordance with the values of bits of gradation data indicating gradation on the pixels for a period corresponding to weighting to the bits in each of subfields of one field.
Pulse-width modulation is performed in accordance with the value of each bit of the gradation data for a bit-turn-on (-off) period in one field, thus controlling the root mean square value of a voltage supplied to the liquid crystal for gradation control.
Bit-turn-on (-off) signals in each subfield are bit data of low or high level, thus not requiring analog processing circuitry, such as, a D/A converter and an operational amplifier.
Therefore, images displayed in this method are free from problems caused by instability of circuit property and wiring resistance, etc., which may otherwise occur when analog processing circuitry is employed.
This method is advantageous in pixel turn-on (-off) control performed per subfield.
Nevertheless, on- (-off) control to a drive voltage that consists of one type of pulse to the liquid crystal could cause input/output gradation differences based on the liquid-crystal response characteristics.
In addition, this method is prone to change in drive characteristics among RGB liquid-crystal panels and also change in gamma characteristics due to change in liquid-crystal response characteristics caused by temperature change, thus disadvantageous in color reproduction.
Pulse-driven adjustments to the drive characteristics among RGB liquid-crystal panels is, for example, disclosed in Japanese Unexamined Patent Publication No. 6-138434 (1994).
A liquid-crystal projector disclosed here is equipped with liquid-crystal panels for R, G and B colors and a driver for controlling a pulse width of a drive pulse signal for driving each of the RGB liquid-crystal panels in accordance with input color signals to be displayed, for adjustments to the drive characteristics different among the R, G and B colors.
The drive-pulse width is varied per RGB liquid-crystal panel based on difference in intensity curves among the panels.
It is well known that chromaticity (x, y) from white to black (dark scale) is constant at (0. 3, 0. 3) while gradation is being varied from white to black with projection of colors with combination of light beams from three liquid-crystal panels, in full-color displaying.
Difference in RGB liquid-crystal panel characteristics could, however, cause displaying of yellow for bright white or purple for dark black.
The drive-characteristics adjustments disclosed in Japanese Unexamined Patent Publication No. 6-138434 (1994) Achieves enhanced gray scale with pulse-width adjustments to drive voltage per RGB panel to adjust drive characteristics.
This technique is advantageous to fewer pixels and also less gradation where as disadvantageous to high-density full-color displaying due to circuit complexity. It is also disadvantageous in easily causing variation in gradation, or difficulty in fine gradation adjustments due to non-linear liquid-crystal driving. Moreover, no anti-high-temperature response-speed measurements are disclosed for projectors that suffer from temperature rise.
Disclosed, for example, in Japanese Unexamined Patent Publication No. 2001-290174 is adjustments to drive parameters against change in liquid-crystal characteristics due to temperature change.
In detail, disclosed here is adjustments to input voltages in accordance with liquid-crystal characteristics per temperature (gamma correction) to achieve fined is playing against temperature change for high response-speed smectic liquid crystals suffering from change in voltage-to-transmissivity characteristics due to temperature change.
Gamma correction is achieved with reconversion of 8-bit digital signal to 8-bit digital signal, adjustments to digital-to-analog conversion characteristics to match voltage-to-transmissivity characteristics and conversion of 8-bit digital signal to 10-bit digital signal.
The technique disclosed in this publication is advantageous in accurate gamma correction with a digital gamma-correction circuit that digitally handles input digital data.
Nevertheless, the output of the digital gamma-correction circuit converted into an analog signal by D/A conversion is prone to noises in driving a liquid crystal.
Moreover, the technique disclosed in this publication requires high costs on temperature sensors attached to liquid-crystal cells for gamma correction. In addition, temperature distribution over the liquid-crystal cells causes difficulty in accurate gamma correction.